Sputtering, alternatively called physical vapor deposition (PVD), is used to deposit several different layers of metals and related materials in the fabrication of semiconductor integrated circuits. In one demanding application, a thin barrier layer is sputtered onto the walls and bottom of a narrow hole etched into and often through an inter-level dielectric layer, most typically composed of silicon oxide or similar oxide materials. The remainder of the hole is then filled with a metal to serve as an electrical connection either vertically in a via penetrating the dielectric layer or horizontally in a trench interconnect formed in the surface of the dielectric layer. The barrier layer prevents diffusion between the metal and the oxide dielectric and thereby prevents oxygen from degrading the metal conductivity and metal from decreasing the resistivity of the dielectric.
In advanced integrated circuits, copper is increasingly used as the metallization material because of its high conductivity and low electromigration. Various barrier materials have been proposed for copper metallization. The most common barrier materials are based upon tantalum, typically in the form of a TaN/Ta bilayer with a TaN layer providing adhesion to the oxide and a Ta layer providing a wetting layer for the copper deposited on it. A magnetron sputter reactor 10 illustrated schematically in cross section in FIG. 1 can effectively sputter thin films of Ta and TaN into holes having high aspect ratios and can further act to plasma clean the substrate and selectively etch portions of the deposited tantalum-based films. The reactor 10 includes a vacuum chamber 12 arranged generally symmetrically about a central axis 14. A vacuum pump system 14 pumps the chamber 12 to a very low base pressure in the range of 10−6 Torr. However, a gas source 18 connected to the chamber through a mass flow controller 20 supplies argon as a sputter working gas. The argon pressure inside the chamber 12 is typically held in the low milliTorr range. A second gas source 22 supplies nitrogen gas into the chamber through another mass flow controller 24 when tantalum nitride is being deposited.
A pedestal 30 arranged about the central axis 14 holds a wafer 32 or other substrate to be sputter coated. An unillustrated clamp ring or electrostatic chuck may be used to hold the wafer 32 to the pedestal 30. An RF power supply 34 is connected to the pedestal 30, which is conductive and acts as an electrode, through a capacitive coupling circuit 36. In the presence of a plasma, the RF biased pedestal 30 develops a negative DC bias, which is effective at attracting and accelerating positive ions in the plasma. An electrically grounded shield 26 protects the chamber walls and the sides of the pedestal 30 from sputter deposition. A target 38 is arranged in opposition to the pedestal 30 and is vacuum sealed to the chamber 12 through an isolator 40. At least the front surface of the target 38 is composed of a metallic material to be deposited on the wafer 32, in this case, tantalum.
A DC power supply 42 electrically biases the target 38 with respect to the grounded shield 36 to cause the argon to discharge into a plasma such that the positively charged argon ions are attracted to the negatively biased target 38 and sputter tantalum from it, some of which falls upon the wafer 32 and deposits a layer of the tantalum target material on it. In reactive sputtering, reactive nitrogen gas is additionally admitted from the nitrogen source 22 into the chamber 12 to react with the tantalum being sputtered to cause the deposition of a tantalum nitride layer on the wafer 32.
The reactor 10 additionally includes an inductive coil 46, preferably having one wide turn wrapped around the central axis 14 just inside of the grounded shield 26 and positioned above the pedestal 30 approximately one-third of the distance to the target 38. The coil 46 is supported on the grounded shield 26 or another inner tubular shield but electrically isolated therefrom, and an electrical lead penetrates the sidewalls of the shield 26 and chamber 12 to power the coil 46. Preferably, the coil 46 is composed of the same barrier material as the target 38. An RF power supply 48 applies RF current to the coil 46 to induce an axial RF magnetic field within the chamber and hence generate an azimuthal RF electric field that is very effective at coupling power into the plasma and increasing its density. The inductively coupled RF power may be used as the primary plasma power source when the target power is turned off and the sputter reactor is being used to etch the wafer 32 with argon ions or for other purposes. The inductively coupled RF power may alternatively act to increase the density of the plasma extending to the target 38. The coil 46 may be composed of tantalum to act as a secondary target under the proper conditions.
The target sputtering rate and sputter ionization fraction can be greatly increased by placing a magnetron 50 is back of the target 38. The magnetron 50 preferably is small, strong, and unbalanced. The smallness and strength increase the ionization ratio and the imbalance projects a magnet field into the processing region for at least two effects of guiding sputtered ions to the wafer and reducing plasma loss to the walls. Such a magnetron includes an inner pole 52 of one magnetic polarity along the central axis and an outer pole 54 which surrounds the inner pole 52 and has the opposite magnetic polarity. The magnetic field extending between the poles 52, 54 in front of the target 38 creates a high-density plasma region 56 adjacent the front face of the target 46, which greatly increases the sputtering rate. The magnetron 50 is unbalanced in the sense that the total magnetic intensity of the outer pole 54, that is, the magnetic flux integrated over its area, is substantially greater than that of the inner pole, for example, by a factor of two or more. The unbalanced magnetic field projects from the target 38 toward the wafer 32 to extend the plasma and to guide sputtered ions to the wafer 32 and reduce plasma diffusion to the sides. To provide a more uniform target sputtering pattern, the magnetron 50 is typically formed in a triangular shape that is asymmetrical about the central axis 14, but a motor 60 drives a rotary shaft 62 extending along the central axis 14 and fixed to a plate 66 supporting the magnetic poles 52, 54 to rotate the magnetron 50 about the central axis 14 and produce an azimuthally uniform time-averaged magnetic field. If the magnetic poles 52, 54 are formed by respective arrays of opposed cylindrical permanent magnets, the plate 66 is advantageously formed of a magnetic material such as magnetically soft stainless steel to serve as a magnetic yoke. Magnetron systems are known in which the radial position of the magnetron can be varied between different phases of the sputtering process and chamber cleaning as described by Gung et al. in provisional application 60/555,992, filled Mar. 26, 2004 and in concurrently filed patent application entitled SELECTABLE DUAL POSITION MAGNETRON, assigned Ser. No. 10/949,735, now issued as U.S. Pat. No. 7,018,515, and incorporated herein by reference in its entirety.
Sputtering tantalum into high aspect-ratio holes requires careful control of the sputtering conditions to balance deposition uniformity over the entire wafer and to achieve good sidewall coverage for both Ta and TaN without etching the top planar surface. The via bottom preferably is left uncoated in vias to provide copper contact between layers while trench bottoms need to remain coated. There are three active species in the sputtering process, neutral tantalum Ta0, tantalum ions Ta+, and argon ions Ar+. Their flux distribution across the wafer in the absence of wafer biasing is shown by plots A, B, C in FIGS. 2, 3, and 4. All these profiles are heavy in the center and light on the edges because of geometric effects arising from the finite spacing between the wafer and the moderately sized target and also because of the tendency of plasma ions to diffuse to the central region of low magnetic field. In the absence of wafer biasing, the net tantalum deposition profile shown in FIG. 5 is the sum A+B of the two tantalum profiles. The center peak can be suppressed somewhat by forming the magnetron to cause heavy sputtering near the target edge. However, this approach causes the sputtered tantalum flux to have a large horizontal velocity component towards the wafer center. The radial particle asymmetry produces an asymmetry between the opposed sidewalls of the hole being coated. Alternatively, wafer biasing can be used to improve the radial deposition uniformity. In particular, the wafer biasing can be optimized to cause the argon ions to partially etch the depositing tantalum. Since the argon profile is also center peaked, the tantalum etching is also stronger at the center. The optimized net tantalum deposition, shown by the profile in FIG. 6, is the sum of A+B−αC, where α depends upon biasing. The profile is not completely uniform but the deposition at the center can be decreased to nearly equal deposition at the edge. If wafer biasing is farther increased, the resultant profile shown in FIG. 7 shows a heavy edge deposition relative to a light center deposition, which is considered worse than the profile of FIG. 6.
Bias optimization by itself has the difficulty that the process window is relatively narrow. Even small variations from the optimized conditions may produce large variations in the net tantalum deposition. Similar types of balancing is required for sidewall coverage, bottom removal, and only partial blanket etching. Heretofore, such balancing has been accomplished primarily by variations of target power, bias power, and coil power. More controlled variables would ease the optimization process and perhaps provide a wider process window.
Auxiliary magnet arrays have been proposed to control sputtering conditions. Permanent magnets have been shown to improve uniformity in the configuration of a simple DC magnetron sputter reactor without an RF coil. However, optimization is still difficult because the magnetization amount is not easily changed. Solenoid coils have also been suggested. While the DC power driving a solenoid coil can be more easily varied, it is still only one additional control. Furthermore, solenoid coils introduce an issue of stray magnetic fields being produced outside the chamber. Sputter reactors are often closely spaced on a cluster tool to other sputter reactors or other types of reactors relying upon tightly controlled magnetic field. Cross effects need to be avoided. A general rule is that stray magnetic fields should not exceed 1 gauss (compared to terrestrial magnetism of about ½ gauss) at 20 inches (50 cm) from the center of the reactor. The distance corresponds generally to the location of a neighboring reactor on a cluster tool The low level of stray field presents a stringent requirement for reactors processing 300 mm wafers.
Electromagnets have been applied to sputtering reactors, as disclosed by Wang in U.S. Pat. No. 6,352,629 and Wang et al. in U.S. Pat. No. 6,730,196. Gung et al. have disclosed two coaxial electromagnets of substantially the same radius for use in a sputter reactor in commonly assigned U.S. patent application Ser. No. 10/608,306, filed Jun. 26, 2003 and now issued as U.S. Pat. No. 7,041,201.
A further consideration is that similar sputter reactors are used in different processes for advanced applications including both tantalum and copper deposition as well as the deposition of some refractory metals. Even for tantalum barrier deposition, different fabrication lines optimize for different designs, and integrated circuit designs evolve over time even for the same manufacturer. It is thus commercially advantageous to produce a sputter reactor that is flexible enough to allow easy process optimization and to be adapted to satisfy the needs of different deposition steps and materials. Yet further, it is known to use a sputter reactor in a number of different modes, for example, with varying wafer bias and varying ionization fractions during different steps of the process. It is possible also to sputter the wafer with argon after the barrier metal has been deposited to remove overhangs or bottom barriers. It would be desirable to provide different magnetic focusing in these different steps.